Systems and method for laser voltage imaging state mapping

ABSTRACT

An apparatus and method for laser probing of a DUT is disclosed. The system enables laser voltage imaging state mapping of devices within the DUT. A selected area of the DUT is illuminating a while the DUT is receiving test signals causing certain of the active devices to modulate. Light reflected from the DUT is collected and is converted into an electrical signal. Phase information is extracting from the electrical signal and a two-dimensional image is generated from the phase information, wherein the two-dimensional image spatially correlates to the selected area.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from U.S. ProvisionalPatent Application No. 61/174,962 filed on May 1, 2009, the entiredisclosure of which is relied upon and incorporated by reference herein.

BACKGROUND

1. Field of the Invention

The present invention relates to an apparatus and method for probingintegrated circuits using laser illumination.

2. Description of the Related Art

Probing systems have been used in the art for testing and debuggingintegrated circuit (IC) designs and layouts. Various laser-based systemsfor probing IC's are known in the prior art. While some description ofthe prior art is provided herein, the reader is encouraged to alsoreview U.S. Pat. Nos. 5,208,648, 5,220,403 and 5,940,545, which areincorporated herein by reference in their entirety. Additional relatedinformation can be found in Yee, W. M., et al. Laser Voltage Probe(LVP): A Novel Optical Probing Technology for Flip-Chip PackagedMicroprocessors, in International Symposium for Testing and FailureAnalysis (ISTFA), 2000, p 3-8; Bruce, M. et al. Waveform Acquisitionfrom the Backside of Silicon Using Electro-Optic Probing, inInternational Symposium for Testing and Failure Analysis (ISTFA), 1999,p 19-25; Kolachina, S. et al. Optical Waveform Probing—Strategies forNon-Flipchip Devices and Other Applications, in International Symposiumfor Testing and Failure Analysis (ISTFA), 2001, p 51-57; Soref, R. A.and B. R. Bennett, Electrooptical Effects in Silicon. IEEE Journal ofQuantum Electronics, 1987. QE-23(1): p. 123-9; Kasapi, S., et al., LaserBeam Backside Probing of CMOS Integrated Circuits. MicroelectronicsReliability, 1999. 39: p. 957; Wilsher, K., et al. Integrated CircuitWaveform Probing Using Optical Phase Shift Detection, in InternationalSymposium for Testing and Failure Analysis (ISTFA), 2000, p 479-85;Heinrich, H. K., Picosecond Noninvasive Optical Detection of InternalElectrical Signals in Flip-Chip-Mounted Silicon Integrated Circuits. IBMJournal of Research and Development, 1990. 34(2/3): p. 162-72; Heinrich,H. K., D. M. Bloom, and B. R. Hemenway, Noninvasive sheet charge densityprobe for integrated silicon devices. Applied Physics Letters, 1986.48(16): p. 1066-1068; Heinrich, H. K., D. M. Bloom, and B. R. Hemenway,Erratum to Noninvasive sheet charge density probe for integrated silicondevices. Applied Physics Letters, 1986. 48(26): p. 1811; Heinrich, H.K., et al., Measurement of real-time digital signals in a siliconbipolar junction transistor using a noninvasive optical probe. IEEEElectron Device Letters, 1986. 22(12): p. 650-652; Hemenway, B. R., etal., Optical detection of charge modulation in silicon integratedcircuits using a multimode laser-diode probe. IEEE Electron DeviceLetters, 1987. 8(8): p. 344-346; A. Black, C. Courville, G Schultheis,H. Heinrich, Optical Sampling of GHz Charge Density Modulation inSIlicon Bipolar Junction Transistors Electronics Letters, 1987, Vol. 23,No. 15, p. 783-784, which are incorporated herein by reference in theirentirety and Kindereit U, Boit C, Kerst U, Kasapi S, Ispasoiu R, Ng R,Lo W, Comparison of Laser Voltage Probing and Mapping Results inOversized and Minimum Size Devices of 120 nm and 65 nm Technology,Microelectronics Reliability 48 (2008) 1322-1326, 19th EuropeanSymposium on Reliability of Electron Devices, Failure Physics andAnalysis (ESREF 2008).

As is known, during debug and testing of an IC, a commercially availabletesting platform, such as, e.g., Automated Testing Equipment, also knownas an Automated Testing and Evaluation (ATE) tester, is used to generatetest patterns (also referred to as test vectors) to be applied to the ICdevice under test (DUT). Various systems and method can then be used totest the response of the DUT to the test vectors. One such method isgenerally referred to as laser voltage probing (LVP). When a laser-basedsystem such as an LVP is used for probing, the DUT is illuminated by thelaser and the light reflected from the DUT is collected by the probingsystem. As the laser beam strikes the DUT, the laser beam is modulatedby the response of various elements of the DUT to the test vectors. Thishas been ascribed to the electrical modulation of the free carrierdensity, and the resultant perturbation of the index of refraction andabsorption coefficient of the material of the IC, most commonly silicon.Accordingly, analysis of the reflected light provides information aboutthe operation of various devices in the DUT.

FIG. 1 is a general schematic depicting major components of alaser-based voltage probe system architecture, 100, according to theprior art. In FIG. 1, dashed arrows represent optical path, while solidarrows represent electronic signal path. The optical paths representedby curved lines are generally made using fiber optic cables. Probesystem 100 comprises a laser source which, in this particular example,is a dual laser source, DLS 110, an optical bench 112, and dataacquisition and analysis apparatus 114. The optical bench 112 includesprovisions for mounting the DUT 160. A conventional ATE tester 140provides stimulus signals and receives response signals 142 to/from theDUT 160 and may provide trigger and clock signals, 144, to the time-baseboard 155. The signal from the tester is generally transferred to theDUT via test boards, DUT board (adapter plate) and various cables andinterfaces that connect all of these components. The time-base board 155synchronizes the signal acquisition with the DUT stimulus and the laserpulses. Workstation 170 controls as well as receives, processes, anddisplays data from the signal acquisition board 150, time-base board155, and the optical bench 112.

The various elements of probe system 100 will now be described in moredetail. Since temporal resolution is of high importance in testingDUT's, the embodiment of FIG. 1 utilizes prior art pulsed lasers,wherein the laser pulse width determines the temporal resolution of thesystem. Dual laser source 110 consists of two lasers: a pulsedmode-locked laser, MLL 104, source that is used to generate 10-35 pswide pulses, and a continuous-wave laser source, CWL 106, that can beexternally gated to generate approximately 1 s wide pulses. The MLL 104source runs at a fixed frequency, typically 100 MHz, and must besynchronized with the stimulus 142 provided to the DUT 160, via aphase-locked loop (PLL) on the time-base board 155, and the trigger andclock signals 144 provided by the ATE tester. The output of the DLS 110is transmitted to the optical bench 112 using fiber optics cable 115.The light beam is then manipulated by beam optics 125, which directs thelight beam to illuminate selected parts of the DUT 160. The beam optics125 consists of a Laser Scanning Microscope (LSM 130) and beammanipulation optics (BMO 135). The specific elements that areconventional to such an optics setup, such as objective lens, etc., arenot shown. Generally, BMO 135 consists of optical elements necessary tomanipulate the beam to the required shape, focus, polarization, etc.,while the LSM 130 consists of elements necessary for scanning the beamover a specified area of the DUT. In addition to scanning the beam, theLSM 130 has vector-pointing mode to direct the laser beams to anywherewithin the field-of-view of the LSM and Objective Lens. The X-Y-Z stage120 moves the beam optics 125 relative to the stationary DUT 160. Usingthe stage 120 and the vector-pointing mode of the LSM 130, any point ofinterest on the DUT 160 may be illuminated and probed.

For probing the DUT 160, the ATE 140 sends stimulus signals 142 to theDUT, in synchronization with the trigger and clock signals provided tothe phase-locked loop on the time-base board 155. The phase-lock loopcontrols the MLL 104 to synchronize its output pulses to the stimulussignals 142 to the DUT. MLL 104 emits laser pulses that illuminate aparticular device of interest on the DUT that is being stimulated. Thereflected light from the DUT is collected by the beam optics 125, and istransmitted to photodetector 138 via fiber optic cable 134. Thereflected beam changes character depending on the reaction of the deviceto the stimulus signal. To monitor incident laser power, for purposes ofcompensating for laser power fluctuations, for example, optical bench112 provides means to divert a portion of MLL 104 incident pulse tophotodetector 136 via fiber optic cable 132. The output signal of thephotosensors 136, 138 is sent to signal acquisition board 150, which, inturn, sends the signal to the controller 170. By manipulation of thephase lock loop on the time-base board 155, controller 170 controls theprecise time position of MLL 104 pulses with respect to DUT 160 stimulussignals 142. By changing this time position and monitoring thephotosensors signals, the controller 170 can analyze the temporalresponse of the DUT to the stimulus signals 142. The temporal resolutionof the analysis is dependent upon the width of the MLL 104 pulse.

It is also known in the art to perform continuous wave LVP, wherein acontinuous wave laser is used to illuminate a device on the DUT and thecontinuously reflected light is collected. The continuously reflectedlight contains timing information relating to the response, i.e.,switching, of the active device to various stimulus signals. Thereflected light signal is continuously converted into electrical signalby a photodetector, e.g., avalanche photodiode (APD) and is amplified.The timing information is contained within the electrical signal andrepresents detected modulation of the device, which can then bedisplayed in either the time-domain using an oscilloscope or in thefrequency domain using a spectrum analyzer.

Recently the technology of laser voltage imaging has been developed toprovide a two-dimensional gray-scale image correlating to voltages atdifferent points in an area of the DUT. More specifically, an LSM isused to raster-scan an area of the DUT and at each point within the areathe reflected light signal is collected and provides a single datavalue. That is, rather than providing the spectra over a range offrequency band, at each point the amplitude of the signal at aparticular frequency spectrum is obtained from the spectrum analyzer. Inpractice, the spectrum analyzer is set to extract a single frequency ofinterest (called zero-span), and to provide an output value that isdirectly proportional to the strength of the received signal at thatfrequency. Consequently, as the LSM scans the selected area of the DUT,if there is no activity at the frequency of interest, the spectrumanalyzer provides low or no output, while if there is activity at thatfrequency, the spectrum analyzer provides high output. That is, thespectrum analyzer provides an output signal whose amplitude isproportional to the strength of the signal at the selected frequency ofinterest. This output can be used to generate a map of the scanned area,showing gray-scale levels corresponding to device activity at each pointin the scanned area.

While the above systems and methods provide valuable information aboutthe functionality of the DUT, it is desirable to non-invasively obtainfurther information about the response of various active devices withinthe DUT.

SUMMARY

The following summary is included in order to provide a basicunderstanding of some aspects and features of the invention. Thissummary is not an extensive overview of the invention and as such it isnot intended to particularly identify key or critical elements of theinvention or to delineate the scope of the invention. Its sole purposeis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented below.

Various embodiments of the present invention provide apparatus andmethod for laser voltage imaging state mapping of a DUT.

An apparatus and method for laser probing of a DUT is disclosed. Thesystem enables laser voltage imaging state mapping of devices within theDUT. A selected area of the DUT is illuminated while the DUT isreceiving test signals causing certain active devices to switch. Lightreflected from the DUT is collected and is converted into an electricalsignal. Phase information is extracted from the electrical signal and atwo-dimensional image is generated from the phase information, whereinthe two-dimensional image spatially correlates to the selected area.

Other aspects and features of the invention will become apparent fromthe description of various embodiments described herein, and which comewithin the scope and spirit of the invention as claimed in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic depicting major components of alaser-based voltage probe system architecture according to the priorart.

FIG. 2 is a diagram illustrating the main component of a systemaccording to an embodiment of the invention.

FIG. 3 is a diagram illustrating waveforms of signals at selected pointsA and B.

FIG. 4 is a diagram illustrating waveforms of signals at selected pointsA and B and their interference with an added interference signal.

FIG. 5 is a diagram illustrating an embodiment of the invention whereinthe ‘RF interference’ signal is added to the “conditioned” signal fromthe APD and is supplied to a spectrum analyzer. In this context,“conditioned” may mean amplified, shifted, converted from current tovoltage and vice versa, etc.

FIG. 6 depicts an embodiment of the present invention that is avariation of the embodiment of FIG. 5.

FIG. 7 depicts another embodiment of the present invention wherein the‘RF interference’ signal is added to the “conditioned” signal from theAPD and is supplied to a spectrum analyzer.

FIG. 8 depicts yet another embodiment of the present invention whereinthe ‘RF interference’ signal is added to the “conditioned” signal fromthe APD and is supplied to a spectrum analyzer.

The invention is described herein with reference to particularembodiments thereof, which are exemplified in the drawings. It should beunderstood, however, that the various embodiments depicted in thedrawings are only exemplary and may not limit the invention as definedin the appended claims.

DETAILED DESCRIPTION

Various embodiments of the present invention provide apparatus andmethod for non-invasive, non-contact method for differentiating therelative polarity of active transistors within a selected area of theDUT, without prior knowledge of the design of the IC. These system andmethod are referred to herein as laser voltage imaging (LVI) statemapping of a DUT. The described methodologies augment the prior artsystem by providing phase information for various active devices withinthe DUT. The phase information can be provided in the form of a map of ascanned area of the DUT, wherein grayscale is used to indicate phaseinformation of active devices, i.e., transistors, that are locatedwithin the scanned area. This enables testing and debug of IC's evenwhen the circuit design is not available.

According to one embodiment of the invention, a lock-in amplifier isused to perform LVI state mapping of an area of interest within the DUT.This embodiment provides the ability to observe relative logic states ofthe various active transistors by extracting phase information from thereflected laser light. According to one embodiment, the lock-inamplifier is used to determine the phase of the reflected signalrelative to a reference signal, which may be generated internally by thelock-in amplifier, or fed externally to the lock-in amplifier. Accordingto one implementation, this is achieved by replacing the spectrumanalyzer of a conventional LVI with a lock-in amplifier.

FIG. 2 is a schematic of a system according to an embodiment of theinvention for performing the phase detection and mapping. In FIG. 2, alock-in amplifier is used in placed of the spectrum analyzer, which isused in the prior art known system. A laser source 210 provides a laserbeam (shown as solid arrow) which is fed into the input fiber optics215. An optical I/O module 214 shapes the beam and provide a conditionedbeam to the LSM 230, which scans the conditioned beam onto a selectedarea of the DUT 260. In this particular example, the path form the LSM230 to the DUT 260 includes a scanning lens, a reflecting mirror, a tubelens, a waveplate, and an objective lens. These elements are provided toproperly scan the laser beam over the selected area of the DUT, butother elements can be used as needed for a particular design.

As the laser beam is scanned over the selected area of DUT 260, astimulus signal 240 is applied to the DUT 260, such that active elementswithin the DUT 260 modulate, i.e., transistors within the DUT switch.The stimulus signal 240 can be generated by a function generator, anATE, etc. As active devices switch, they change the absorptioncoefficient and the refractive index of the material making the device,e.g., silicon, such that the amplitude of the reflected laser beam(shown as broken arrow) is modulated correspondingly. The reflected beamis collected by the optical elements and directed onto the output fiber232, which directs the beam onto a sensor. In this particular example anavalanche photodiode APD 236 is used, but other photosensors can beused, such as, e.g., PIN sensor. The output signal of the APD is inputto trans-impedance amplifier 237 and the output of the TIA is input intoa signal separator, such as a bias-tee (diplexer) 250, which outputs aDC component and an AC component. The DC component is amplified by thevideo amplifier 252 and is sent to the frame grabber 254 for generatingan image of the scanned area of the DUT. The AC component (at RFfrequency) is conditioned by RF amplifier 273 and is then sent to thelock-in amplifier 270. The output of the lock-in amplifier 270 is alsoamplified by a video amplifier 256 and is used to generate a phase imageof the scanned area. As will be described more fully below, the X/Y orR/Θ output of the lock-in amplifier is converted into a gray scale imageof the scanned area, wherein the values of the gray scale represent thephase of active devices in the scanned area of the DUT.

The operation of an embodiment of the invention that utilizes a lock-inamplifier will now be described. The X and Y values of the lock-inamplifier are proportional to the amplitude and relative phase of thesignal, i.e.,XαV _(sig) cos ΘYαV _(sig) sin ΘWherein V_(sig) is the amplitude of the signal of interest (reflectedlaser beam), Θ is the phase difference between the signal of interestand a reference signal (e.g., a reference clock signal), i.e.,Θ=Θ_(sig)−Θ_(ref). For a pair of transistors modulated at oppositestates or polarity, the X or Y output values would be at oppositepolarities, regardless of the phase of the input reference frequency.For example, if transistor A is modulating at Θ₁, then transistor B ismodulating at Θ₂=Θ₁+/−180° (out of phase). Therefore, the X value fortransistor A is proportional to cos Θ₁, while the X value for transistorB is proportional to cos Θ₁+/−180°. That is:X _(A) αV _(sig) cos Θ₁X _(B) αV _(sig) cos(Θ₁+/−180°)=−V _(sig) cos Θ₁ =−X _(A)Similarly,Y _(A) αV _(sig) sin Θ₁Y _(B) αV _(sig) sin(Θ₁+/−180°)=−V _(sig) sin Θ₁ =−Y _(A)Therefore, the relative logic states can be extracted from the lock-inamplifier's X or Y output. It should be noted, however, that this schemeis not limited to in phase and out of phase detection. Rather, so longas the phase difference between the two transistors is larger thanninety degrees, the X and Y values of these two transistors will be ofopposite polarity, albeit at different absolute amplitude. The X or Youtput of the lock-in amplifier may be converted to gray-scale image,wherein the values of each pixel corresponds to the phase at thatspatial location.

According to another embodiment, a combination of the lock-inamplifier's R and Θ values are used. According to this embodiment:R=V _(sig)=√(X ² +Y ²)Θ=tan⁻¹(Y/X)Θ is the phase difference between the signal of interest and a referencesignal. However, when the laser beam scans over an area of the IC wherethere are no transistors, there is no reflected RF electrical signal andthe Θ value is random. Consequently, the Θ output voltage of the lock-inamplifier is random, which will be seen as noise. This can cause the Θvalue coming from the transistors to be masked by the Θ “noise”.Therefore, according to one embodiment the R output is monitored todetermine whether the Θ output voltage value should be used or not,i.e., whether is Θ value is random or not. A reflected RF electricalsignal will result in a non-zero value for R, which in turn allows the Θvalue to be used for that particular pixel in the scanned area of theIC. On the other hand, a non-existing reflected RF electrical signalwill give an almost zero value to R, which in turn disallows the use ofthe Θ value for that particular pixel. In one example, a threshold isset for the amplitude of R value which allows/disallows the use of the Θvalue.

According to the above embodiment, for a pair of transistors modulatingat opposing states, the difference in the Θ value would be a constant180 degrees (ΔΘ=Θ_(A)−Θ_(B)=180°) regardless of the phase of the inputreference frequency. A lock-in amplifier would typically output ananalog voltage of +/−V to correspond to the measured phase difference of+/−180°. Since the phase difference is 180°, the analog voltageamplitude difference would be V (ΔV=V_(A)−V_(B)=V). The relativepolarity between the two transistors can be then extracted by setting athreshold value that is between V_(A) and V_(B) using various methods.

According to various other embodiments of the invention, the ability toobserve relative logic states in the LVI is manifested by theintroduction of ‘RF interference’ into the acquisition system, andsupplying the resulting signal to a spectrum analyzer. The termacquisition system is meant to include any one or combination of theAPD, the TIA, the Bias-Tee, the RF amplifier, and the spectrum analyzer,i.e., the ‘RF interference’ may be coupled into any of these or at anypoint in their connections. Herein, we refer to the frequency spectra ofthis ‘RF interference’ as ‘interference’ spectrum and it served asomewhat similar function to the reference signal in the embodiment ofFIG. 2. In the following embodiments the use of a swept-tuned,superheterodyne spectrum analyzer is illustrated, but similar resultscan be achieved using other means, such as real-time spectrum analyzer(also called FFT spectrum analyzer), vector signal analyzer, etc.

For effective results, the ‘RF interference’ should be at the samefrequency and be synchronous with the internal signals under analysis.If it meets these requirements, this ‘RF interference’ will interfereeither constructively or destructively with the detected modulation(converted from optical to electrical by the acquisition system) of thetransistors carrying the internal signals under analysis. If thedestructive interference brings the amplitude of the electrical signalbelow the amplitude of the electrical signal of ‘RF interference’ alone,the resulting spectra would have less energy than the ‘interference’spectra. Phase shifting of the ‘RF interference’ signal may be done toensure that the ‘RF interference’ signal is in-phase and out-of-phasewith the signals of interest for optimal/maximum constructive anddestructive interference.

To illustrate, reference is now made to FIG. 3, showing waveforms ofsignals at selected points A and B. Assume for this example that point Aand point B are the same instances of an inverter, connected in series.This means that the signals at point A and point B are out-of-phase oropposite logic states relative to each other. The modulation detected bythe acquisition system is illustrated in FIG. 3, although in reality thesignal level would be very low, between sub-microvolts to hundreds ofmicrovolts, and requires averaging to achieve desired SNR. In thisillustration, the term ‘signal’ refers to the electrical signal of thedetected optical modulation. To a conventional spectrum analyzer, sinceboth waveforms have the same amplitude, the power of the spectra at thefrequency of interest is the same—there is no differentiation betweenpoints A and B.

Using embodiments of the invention, if ‘RF interference’ electricalsignal, at the same frequency and also in synchronous with the abovemodulated signal, is introduced to the acquisition system, theelectrical signals of the detected modulation at points A and B willinterfere with this introduced signal. Such situations are illustratedin FIG. 4. If the ‘RF interference’ signal, shown as f_(int) and havingamplitude x a.u., interferes with the signal at point A (also havingamplitude x a.u.), the resulting electrical signal would have addedamplitude, i.e., 2 x a.u., as shown by waveform Σ_(f+A). On the otherhand, if the interference signal, f_(int), interferes with the signal atpoint B, the resulting electrical signal would be a null, i.e., 0 a.u.,as shown by waveform Σ_(f+B). Therefore, there will be three differentamplitudes at the spectrum of interest that the spectrum analyzer willmeasure.

2x a.u at point A

x a.u. where there is no activity (only RF interference signal ismeasured)

0 a.u at point B

Normalizing this into a grayscale level, one would see white pixels atpoint A, gray pixels at points with no activity (background level), andblack pixels at point B, hence providing a relative logic state mappingbetween point A and B.

As noted above, the introduction of an interference signal can be doneat different points of the acquisition system. The ‘RF interference’signal may be collected by a variety of ways, e.g., through anelectrical connector or picked up by an antenna as there will be someamount of ‘RF interference’ electromagnetic waves emitted from the testcell (stimulus, DUT, etc). The ‘RF interference’ signal may then becoupled into the acquisition by a variety of ways, e.g., using a summingamplifier/voltage adder or through intentional transmission of ‘RFinterference’ electromagnetic waves or through a simple electricalT-connection.

Regardless of the collection and coupling of the ‘RF interference’, thecollected ‘RF interference’ signal needs to be gain conditioned (insimple terms based on the illustrations above). A programmable RFamplifier is required to either boost or attenuate the amplitude of thecollected ‘RF interference’ signal, depending on how the signal wascollected. The ‘RF interference’ signal may be also phase-conditioned toallow for maximum interference. One might require phase shifting the ‘RFinterference’ signal if the gain-conditioning cannot achieve sufficientconstructive or destructive interference due to the ‘RF interference’signal having a slight phase-shift relative to a particular signal underanalysis.

FIG. 5 illustrates an embodiment of the invention wherein the ‘RFinterference’ signal is collected through cables (electrical signal) orantenna (RF electromagnetic waves). The RF interference may be collectedfrom the DUT, the tester (e.g. ATE), a tester board, a DUT board, orcables that interface these components. The ‘RF interference’ signal isconditioned (gain/attenuate and phase shift) before being added with asumming amplifier or voltage adder after the RF amplifier. Theillumination and beam reflection collection parts of the embodiment ofFIG. 5 are similar to that of FIG. 2 and would therefore, not bedescribed here again. What follows is a description of the elements thatare different from the embodiment of FIG. 2. Most notable, the lock-inamplifier of FIG. 2 if replaced by a spectrum analyzer 572 in FIG. 5.However, in order to enable the spectrum analyzer to detect and generatea signal indicative of phase, the following elements are added. Namely,an interference signal is collected from an antenna 580 or a cable 582(note that while both antenna and cable are shown in this embodiment,this is for illustration only and one may include only one or the otheror both). The interference signal is conditioned, i.e., amplified orattenuated, by the signal conditioner 571 and is then phase shifted byphase shifter 570. The conditioned interference signal is then input toa summing amplifier or voltage adder 574, to be added to the conditionedsignal of the APD. The output is then provided to the spectrum analyzer572. The output of the spectrum analyzer is provided to a videoamplifier, which provides its signal to a data acquisition module. Inthis example, a frame grabber is used to generate a gray scale imagemapping indicative of the phase of the active elements within thescanned area of the DUT. Of course, any other data acquisition card ormodule can be used.

FIG. 6 depicts an embodiment of the present invention that is avariation of the embodiment of FIG. 5. Notably, in FIG. 6 the additionof the ‘RF interference’ signal is done before the RF amplifier 273.That is, the conditioned interference signal is added to the RF signalfrom the bias-tee 250 by the summing amplifier or voltage adder 674. Theoutput of the adder 674 is then amplified by RF amplifier 273 and isthen input to the spectrum analyzer.

FIG. 7 depicts an embodiment of the present invention that is avariation of the embodiment of FIG. 5. Notably, in FIG. 7 the additionof the ‘RF interference’ signal is done by radiating the interferencesignal onto the electrical path of the APD signal. That is, theconditioned interference signal is applied by the RF gain/attenuator 570and/or phase shifter 571 to an antenna 700. The antenna 700 is placedsuch that its radiation would be directed to the electrical path of thesignal from the APD and be detected by and interfere with the signal ofthe TIA 237, Bias Tee 250 and/or amplifier 273. In this manner, theinterference signal is added onto the signal that is input to thespectrum analyzer 572.

FIG. 8 depicts yet another embodiment of the present invention that is avariation of the embodiment of FIG. 5. Notably, in FIG. 8 the additionof the ‘RF interference’ signal is done by coupling the interferencesignal onto the conditioned APD signal using a T-connection coupler.That is, the conditioned interference signal is applied to T-connection874 which also receives the conditioned signal from amplifier 273. Inthis manner, the interference signal is added onto the signal that isinput to the spectrum analyzer 572.

While the invention has been described with reference to particularembodiments thereof, it is not limited to those embodiments.Specifically, various variations and modifications may be implemented bythose of ordinary skill in the art without departing from theinvention's spirit and scope, as defined by the appended claims.Additionally, all of the above-cited prior art references areincorporated herein by reference.

What is claimed is:
 1. A method for state mapping of active deviceswithin a device under test (DUT), comprising: illuminating a selectedarea of the DUT while the DUT is receiving test signals causing certainof the active devices to modulate, the selected area having at least twoof the active devices situated therein and the two active devicesmodulating at opposing logic states; collecting reflected light from theselected area; converting the reflected light into an electrical signal;extracting relative phase information of the two active devices from theelectrical signal to thereby extract the logic states of active deviceswithin the DUT; generating a two-dimensional image from the phaseinformation, wherein the two-dimensional image spatially correlates tothe selected area.
 2. The method of claim 1, wherein extracting phaseinformation comprises mixing the electrical signal with an interferencesignal.
 3. The method of claim 2, wherein mixing comprises applying theelectrical signal and an interference signal to a spectrum analyzer. 4.The method of claim 3, further comprising collecting electromagneticradiation emitted by at least one of the DUT, a tester, a test board, aDUT board, and cables to generate the interference signal.
 5. The methodof claim 4, further comprising conditioning the interference signalprior to the mixing.
 6. The method of claim 5, wherein conditioningcomprises at least one of: amplifying, attenuating, and phase shifting.7. The method of claim 3, wherein mixing comprises combining theelectrical signal and the interference signal in one of a summingamplifier and a voltage adder.
 8. The method of claim 7, furthercomprising RF amplifying the electrical signal prior to combining. 9.The method of claim 3, wherein mixing comprises applying the electricalsignal and the interference signal to a T-connector.
 10. The method ofclaim 3, wherein generating a two-dimensional image comprises applyingoutput of the spectrum analyzer to a data acquisition module.
 11. Themethod of claim 2, wherein mixing the electrical signal with aninterference signal comprises mixing the electrical signal with an RFinterference signal.